1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to an electrolyte containing additives useful for stabilizing transition metal cyanometallate batteries.
2. Description of the Related Art
Transition metal cyanometallates (TMCMs) with large interstitial spaces have been investigated as the cathode material for rechargeable lithium-ion batteries [1, 2], sodium-ion batteries [3, 4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)-HCFs) exhibited a very good cycling life with 83% capacity retained after 40,000 cycles at a charge/discharge current of 17 C [6-8]. However, the materials within the aqueous electrolyte demonstrated low capacities and energy densities because: (1) just one sodium-ion can be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) these transition metal cyanoferrate (TM-HCF) electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction.
To correct the shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in a non-aqueous electrolyte [9, 10]. Assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V and delivered capacities of about 110 milliamp hour per gram (mAh/g).
It is worth noting that the actual capacity of a TMHCF electrode is by far smaller than the theoretical value. For instance, the theoretical capacity for Mn-HCF is 170 mAh/g, but the reported capacity was just ˜120 mAh/g, as tested in a sodium-ion battery. The capacity difference could be ascribed to the structures and compositions of TMHCFs. Buser, et al. [11] investigated the crystal structure of Prussian Blue (PB), Fe4[Fe(CN)6]3.xH2O and found that Fe(CN)6 positions were only partly occupied. The vacancies led to water entering the PB interstitial space and even associating with Fe(III) in the lattice [12]. In consideration of charge neutralization and interstitial space, the vacancies and water both act to reduce the concentration of mobile ions in the interstitial space of TMHCF. As an example, Matsuda, et al. [9] preferred to use A4x-2MA[MB(CN)6]x.zH2O as a replacement to the nominal formula of A2MAMB(CN)6 because of the vacancies. Furthermore, the vacancies result in dense defects on the surface of TMHCFs. Without interstitial ions and supporting water, the surface easily collapses. The surface degradation can be aggravated when the interstitial ions in the vicinity of the surface are extracted out during electrochemical reactions. In a battery, such degradation leads to poor capacity retention.
A Cu-HCF electrode with a Li+-Lion electrolyte delivered 120 mAh/g during the first discharge, but its capacity decreased to 40 mAh/g in 10 cycles [13]. By coating with Ni-HCF, the surface of the Cu-HCF electrode was modified and its stability was improved. However, the undercoordinated transition metal (UTM) on the surface retards charge transfer between the TMHCF electrode and electrolyte due to charge repulsion between the UTM and the mobile ions, which may result in poor rate performance. Park, et al. [14] mentioned the surface effect on a LiFePO4 electrode with undercoordinated Fe2+/Fe3+ at the surface creating a barrier for Li+ transport across the electrolyte/electrode interface. To improve the capacity retention, some researchers have optimized the synthesis of TMHCFs to reduce defects and vacancies on their surfaces and in the bulk of the material [15, 16]. These defect-free TMHCFs demonstrated a longer cycle life. However, defects and vacancies in TMCM electrode also likely appear during the charge and discharge cycles of a battery, and they are impossible to prevent by synthesis.
It would be advantageous if additives could be included in an electrolyte that would interact and coordinate with surface of TMCM electrodes, to cure and reduce the defects and undercoordinated metal-ions, and to improve the cycle lifetime.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.    [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48(2012)6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52(2013)1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2 (2011) 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letters, 11 (2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159 (2012) A98-A103.    [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.    [10] S.-H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2(2013)A39-A41.    [11] H. J. Buser, D. Schwarzenhach, W. Petter, A. Ludi, the crystal structure of Prussian blue: Fe4[Fe(CN)6]3.xH═O, Inorganic Chemistry, 16(1977) 2704-2710.    [12] F. Herren, P. Fischer, A. Ludi, W. Hälg, Neutron diffraction study of Prussian blue, Fe4[Fe(CN)6]3.xH2O. Location of water molecules and long-range magnetic order, Inorg. Chem. 1980, 19, 956-959    [13] D. Asakura, C. H. Li, Y. Mizuno, M. Okubo, H. Zhou, D. R. Talham, Bimetallic cyanide-bridged coordination polymers as lithium ion cathode materials: core@shell nanoparticles with enhanced Cyclability, J. Am. Chem. Soc., 135(2013)2793-2799.    [14] K.-S. Park, P. Xiao, S.-Y. Kim, A. Dylla, Y.-M. Choi, G. Henkelman, K. J. Stevenson, J. B. Goodenough, Enhanced charge-transfer kinetics by anion surface modification of LiFePO4, Chem. Mater. 24(2012)3212-3218.    [15] X. Wu, W. Den, J. Qian, Y. Cao, X. Ai, H. Yang, Single-crystal FeFe(CN)6 nanoparticles: a high capacity and high rate cathode for Na-ion batteries, J, Mater. Chem. A., 1(2013)10130-10134.    [16] Y. You, X.-L. Wu, Y.-X. Yin, Y.-G, Guo, High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries, Energy & Environmental Science, Doi: 10.1039/C3EE44004D.